Okay, I don't know why it's making a lighter version, but I think I know a way to fix it.
Single-click on the mask thumbnail in the layers panel. Then press Cmd L to open a Levels window. Grab the middle slider and move it ALL the way to the left.
Stop wasting your time with that stupid function, period. Just mask it like a normal person. https://www.damiensymonds.net/2010/10/shift-while-brushing.html